The present invention relates to an optical examination apparatus for optically examining density, distribution, etc. of oxygen in an object to be examined such as organic tissue like brain tissue, of man or animal.
U.S. Pat. No. 4,281,645 discloses one such technique, in which light emitted from a laser light source is led to brain tissue through an optical fibre bundle, and light is transmitted and scattered by the brain tissues. The laser light source emits near infrared light and may operate at a number of different wavelengths and then the measurement is made at every wavelength.
However, a laser light source, particularly a semiconductor laser, has a problem in that it is not easy to stabilize its output power and variations in the output power are different at different wavelengths, so that the reliability of the result of examination by means of the examination apparatus is lowered. To overcome this problem there has previously been proposed in Japanese Patent Application Unexamined Publication No. 275329/1988 sampling the light output by the light source and using this to normalize the output of the detector which detects the transmitted and scattered light. This arrangement is shown in FIG. 4.
As shown in FIG. 4, an apparatus to measure transmitted and scattered light P.sub.A from organic tissues 1 comprises a laser light source 2, a transmitted and scattered light detector 3, a monitoring light detector 4, and a normalizing device 5. The light emitted from the laser light source 2 is incident on an optical fibre bundle 6 at one end so that the light is partly led to the organic tissues 1 through a first branch fibre bundle 61, and is partly led to the monitoring light detector 4 through a second branch fibre bundle 62. The transmitted and scattered light P.sub.A produced from the organic tissues 1 is led to the transmitted and scattered light detector 3 through another optical fibre bundle 7. The respective outputs detected by the transmitted and scattered light detector 3 and the monitoring light detector 4 are transferred in the form of electric signals to the normalizing device 5. In the normalizing device 5, the transmitted and scattered light output S.sub.A is normalized in accordance with the monitored light output S.sub.M. In the past, When a number of, for example, four, laser light sources 2.sub.1 to 2.sub.4 each having a distinct wavelength .lambda..sub.1 to .lambda..sub.4 ,respectively, are provided, a corresponding number (four) of optical fibre bundles 6.sub.1 to 6.sub.4 are provided as shown in FIG. 5. Thus, the output power of the laser light source 2 is monitored and the transmitted and scattered light output S.sub.A is normalized in accordance with the monitored output power, so that the unevenness due to variations in output in the results of measurements can be compensated for once the apparatus is calibrated for all powers and wavelengths.
Examples of the optical fibre bundle 6 which may used in the conventional apparatus are shown in FIGS. 6 and 7. FIG. 6 shows an optical fibre bundle in which two optical fibres are combined. As shown in the drawing, the laser light emitted from the laser chip 22 fixed to a laser casing 21 is transmitted through a glass window 23. The transmitted light is focused by lenses L.sub.1 and L.sub.2 to be incident on an optical fibre 68 provided at their focus, and then propagated toward the organic tissue 1. On the other hand, another optical fibre 69 is provided adjacent the optical fibre 68, so that laser light displaced from the focus point is incident on that optical fibre 69. Accordingly, if the light propagated through the optical fibre 69 is monitored, the normalization described above can be carried out.
FIG. 7(a) shows another optical fibre bundle in which a number of optical fibres are combined. As seen in the drawings, the optical fibre bundle 6 is split into a first branch fibre bundle 61 for propagating light to the organic tissues and a second branch fibre bundle 62 for propagating light to the monitored light detector 4. The ends of the plurality of optical fibres constituting the second branch fibre bundle 62 are however unevenly distributed over the end face at a portion inside a coating 63 to one side as represented by the circle shown in FIG. 7(b).
In the light source 2, particularly when this includes a semiconductor laser however, variations may occur not only in its output power but in its luminous pattern. That is, in such a semiconductor laser 90 as shown in FIG. 8, laser light is emitted from a stripe region 92 of an active layer 91 with a luminous pattern 93 having a substantially eliptical shape. We have discovered, as shown in FIG. 9, that when the second monitoring branch fibre bundle 62 is located to one side of the optical fibre bundle 6 the overlap between the output and the input ends of the monitoring branch fibre bundle 62 varies. That is, when the luminous pattern 93.sub.1 has a large oval shape as shown in FIG. 9(a), the monitoring output is large, but when the luminous pattern 93.sub.3 has a small oval shape as shown in FIG. 9(c), the monitoring output is small.
FIG. 10 shows the ratio between the output characteristic of the monitoring or second branch fibre bundle 62 with respect to that of the first bundle 61. In the drawing, the axis of ordinate shows the output of the first branch fibre bundle 61 for introducing light to organic tissues, and the axis of abscissa shows the output of the second branch fibre bundle 62 for monitoring. The graph illustrates how the measured data is shifted from a linear relationship where the solid line is a regression line of the measured data.